1. Field of the Invention
The present invention relates to the field of wireless communications, and more particularly, to polarization-diversity systems for wireless communications.
2. Description of the Related Art
It can be fairly said that the age of wireless communications began in 1898 when Guglielmo Marconi broadcast the first paid radio program from the Isle of Wight. The system used by Marconi was a one-way wireless communication system comprising a transmitter that sent messages, carried by electromagnetic waves, to one or more receivers. One-way communications systems, such as broadcast radio, television, etc., are still widely used today.
In contrast to one-way systems that can only send messages from one person to another, duplex (two-way) wireless communications systems, such as cellular telephones, cordless telephones, etc., allow two-way communication between two or more parties. In its simplest form, a duplex communication system is the combination of two one-way systems. In a duplex communication system, each party is equipped with a transceiver (a transmitter combined with a receiver) so that each party can both send and receive messages. Communication is two-way because each transceiver uses its transmitter to send messages to the other transceivers, and each transceiver uses its receiver to receive messages from the other transceivers.
As with normal conversation between people, duplex communication systems typically use some technique to minimize the interference that occurs when two parties try to transmit (i.e., talk) at the same time. As with normal conversation, many duplex systems use some form of a Time Division Duplexing (TDD) algorithm, wherein only one party at a time is allowed to transmit. Each party transmits only during its allotted time interval, and during that time interval, all other parties are expected to receive the transmission (i.e., listen). Other division techniques, such as, for example, frequency division, code division, etc., are also used to separate transmissions between parties.
TDD systems include the Digital European Cordless Telephone (DECT), the Personal Handy phone System (PHS), the Personal ACcess System (PACS), and the Personal Wireless Telecommunications (PWT) system. DECT is a 2nd generation cordless telephone standard, designed to be capable of supporting very high traffic densities at 1895-1906 MHz (private) and 1906-1918 MHz (public), with a proposed extension to a 300 MHz frequency band. DECT uses a TDMA/TDD access technique and a GMSK modulation technique, making it suitable for low mobility-high capacity concentrated usage environments such as city center offices and transport hubs. PHS, developed in Japan, operates at 1880-1900 MHz, uses a TDMA/TDD access technique and a xcfx80/4 QPSK modulation technique. PACS, developed by Bellcore, uses both TDMA/FDD (Frequency Division Duplex) and TDMA/TDD. PWT is the new name for the licensed DT1900 as well as the unlicensed WCPE cordless technologies found in the United States.
In both one-way and duplex communication systems, the transmitter provides Radio Frequency (RF) signals to a transmitting antenna that converts the RF signals into ElectroMagnetic (EM) waves. The EM waves propagate to a receiving antenna where the EM waves are converted back into RF signals that are provided to the receiver. Ideally, the EM waves travel in a single path directly from the transmitting antenna to the receiving antenna, without any external influences or perturbations, and without taking multiple paths. Unfortunately, ideal conditions are rarely found in the real-world and thus the EM waves that propagate from the transmitting antenna to the receiving antenna are often disturbed by external influences. These disturbances often reduce the strength of the EM waves that reach the receiving antenna, and thus impair the performance of the communications system. Fluctuation in the strength of the received signal is known as signal fading. The impairment caused by signal fading can include reduced range, higher noise, higher error rates, etc. Fading is usually caused by destructive interference of multipath waves. In theory, the reduction in signal strength at the receiving antenna can be offset by increasing the strength of the EM wave produced by the transmitting antenna. However, the strength of the EM wave produced by the transmitting antenna is usually limited by various factors, including, government regulations, the size/cost/weight of the transmitter, the size/cost/weight of the transmitting antenna, and the power available to operate the transmitter. The power available to the transmitter is particularly important in battery operated devices, such as handheld cellular telephones, where battery life is an important aspect of overall system performance.
Two common types of signal fading are multipath fading and polarization mismatch fading. Multipath fading occurs when the EM waves take two or more paths to travel from the transmitting antenna to the receiving antenna. The waves arriving at the receiving antenna along different paths will often interfere with each other, such that a wave arriving from a first path will tend to cancel a wave arriving from a second path. Receive-antenna position-diversity is a method often used to mitigate the effects of multipath fading. In systems with receive-antenna position-diversity, several receiving antennas are positioned such that the phase centers (i.e., positions) of the antennas are physically separated by a few wavelengths. The receiving antennas are used to receive the EM waves, and the output from each receiving antenna is provided to the receiver for special processing. Receive-antenna position-diversity works because the destructive interference is typically a localized phenomenon. Even if one of the receiving antennas is experiencing multipath fading, it is likely that another receiving antenna located several wavelengths away will not experience fading. The separation between the antennas is desirable because the probability of having all of the received signals for all of the receiving antennas faded at one time becomes increasingly small as the number of antennas are increased.
Receive-antenna position-diversity is commonly used in wireless base stations where antenna size, weight, and cost are less important than in handheld units. Antenna position diversity is rarely used in handheld units because of the size, weight, and cost associated with multiple receiving antennas spaced several wavelengths apart. For example, conventional analog cellular telephones operate using EM waves having a frequency of approximately 1 GigaHertz (GHz). A 1 GHz EM wave in air has a wavelength of approximately 1 foot. Thus, an effective position-diversity antenna system would be several feet across. This is clearly impractical for a handheld telephone, but very practical for a base station antenna mounted on a large tower.
Various techniques are used to process the antenna outputs, including, for example, Antenna Switching Diversity, and Maximal Ratio Combining. Antenna Switching Diversity systems simply pick the receiving antenna that is currently receiving the strongest EM wave and use that antenna as the receiving antenna.
Maximal Ratio Combining systems combine the outputs of one or more receiving antennas into a single output signal. The outputs of the antennas are coherently phased and weighted to provide maximum power in the output signal. Maximal Ratio Combining typically offers better performance than Antenna Switching Diversity because it combines the antenna outputs, thus bringing in more signal while tending to average out the noise. This results in a higher Signal-to-Noise Ratio (SNR).
The combination of antenna-position diversity and maximal ratio combining is closely related to the technique of antenna-pattern diversity. In antenna pattern diversity, the antenna typically comprises several antenna elements. The transmitter provides RF signal to each antenna element such that the EM radiation from the antenna elements is focused in a particular direction, much like the focused beam from a flashlight. In some locations, such as Japan, regulatory constraints favor the less effective technique of antenna-switching rather than maximal ratio combining. In the Japanese PHS system for example, so-called xe2x80x9csmart antennasxe2x80x9d which provide antenna-pattern diversity, are only allowed if they also reduce the maximum power output provided by each antenna element by an amount proportional to the number of antenna elements. For example, if four antenna elements are available, the maximum output at each antenna element is limited to one-fourth of the legally mandated maximum output power from a single antenna element. A possible rationale for this regulation is that the Japanese PHS system allows competitive service providers to share the same frequency bands. If one competitor is allowed to focus EM waves in one direction, then a nearby base station operated by another competitor, and servicing mobile users along the same radiation path, would experience interference. By reducing the maximum power available to each antenna element in an array of antenna elements, the total power output of the array is limited. This, unfortunately, greatly reduces the effectiveness of transmit diversity using antenna combining by up to 3 dB for a two-antenna system, and up to 6 dB for a four-antenna system. With these constraint losses, antenna-switching tends to outperform maximal ratio combining (at least from a diversity reception standpoint; maximal ratio combining does reduce the interference seen by other users not in the paths of its beams).
Polarization mismatch fading occurs when the polarization of the EM wave that arrives at the receiving antenna does not match the polarization of the receiving antenna. For example, polarization mismatch fading is common when using a mobile handset because different users will orient the handset at different angles. Base station antennas are typically designed for a vertically oriented linear polarization. Most typical handheld units have a small whip antenna (more precisely, a monopole antenna) that is also linearly polarized, with a polarization vector that is parallel to the antenna. Thus, in theory, most handheld units provide the least polarization mismatch fading when the antenna is held vertically. Unfortunately, the wireless handset is rarely held so that the antenna is vertical. The handset is usually held diagonally so that the mouthpiece (microphone) is close to the user""s mouth, and the earpiece (loudspeaker) is over the user""s ear. If the user is standing or sitting, the vertical axis of the mobile handset is therefore often 45 degrees or more off of true vertical. If the user is reclining, the handset may be almost completely horizontal.
Polarization mismatch fading often occurs when the user orients the handheld unit so that the antenna is not vertical. This polarization mismatch fading sometimes goes unnoticed because most communication systems are designed with a power budget that provides a large excess power margin. By holding the antenna at less than optimal orientation, the user is merely unconsciously using up some of the power budget designed into the system. However, at the far fringe of a reception area, most of the power budget is used up just getting the EM waves from the transmitter to the receiver. Thus, at the fringe of a reception area, the user will notice the effects due to polarization mismatch.
Assuming line of sight propagation, a 45 degree polarization mismatch between a single base station antenna and mobile unit antenna results in only half of the power (3 dB) being delivered to the receiver; a 90 degree mismatch results in (theoretically) no power being delivered to the receiver.
Many studies have been done on signal strength versus antenna orientation in the mobile unit. For example K. Li and S. Mikuteit, xe2x80x9cCharacterization of Signal Polarization Near 900 MHz in and on Vehicles and Within Buildingsxe2x80x9d, Proceedings of ICUPC 1997, pp. 838-842, indicates that, indeed, a mobile unit antenna oriented toward the vertical tends to offer higher performance than those oriented toward the horizontal. However, this study also found that in complex, non-line-of-sight (e.g., multipath) environments, the difference between the horizontal and vertical polarization signal strengths can be small. Moreover, in strong multi-path conditions, the above study reports that a circularly polarized antenna (which mixes horizontal and vertical polarizations) performs best. Size, cost, and complexity considerations typically prohibit the incorporation of a circularly polarized antenna into the handset. Likewise, cost, antenna switching losses, and antenna separation considerations tend to disfavor the incorporation of multiple antennas into the handset.
Recently, receive-only base station antenna polarization diversity has been investigated in the hope of improving performance of the path from a handset to a base station such as a cellular tower. This path is often called the uplink. Unfortunately, in the receive-only context, perceived gains have been seen, but they are not sufficient to justify receive-only diversity in many applications. M. Nakano, T. Satoh, and H. Arai, xe2x80x9cUp Link Polarization Diversity and Antenna Gain Measurement of a Hand-Held Terminalxe2x80x9d, IEEE Antennas and Propagation Society International Symposium, Jun. 18-23 1995, vol. 4 pp. 1940-1943, describes the results of field experiments on the received polarization of 900 MHz signals. This paper notes that the average signal level of the horizontal (H) polarization component received from a handheld phone is, in general, greater than the vertical (V) component. Moreover, the paper indicates that the correlation coefficient between horizontal and vertical signals under fading conditions is less than 0.3, which is important since the diversity antennas should be as uncorrelated as possible in order to reap maximum gains.
A. Turkmani, A Arowojolu, P. Jefford, and C. Kellet, xe2x80x9cAn Experimental Evaluation of the Performance of Two-Branch Space and Polarization Diversity Schemes at 1800 MHzxe2x80x9d, IEEE Transactions on Vehicular Technology, vol. 44, no. 2, May 1995, pp. 318-326, describes results similar to Nakano et al., but using 1800 MHz signals. Turkmani et al. concluded that receive-only polarization-diversity outperforms receive-only position diversity. In particular, Turkmani et al. found that a 45-degree oriented handset induced mismatch losses averaging 6 dB, while using two vertical antennas for receive-only antenna-position diversity. By contrast, Turkmani et al. found that a polarization-diverse receiver setup suffered less fading, and showed that the total advantage of using receive-only polarization diversity appears to be approximately 6 dB when the handset is tilted at 45 degrees.
K. Cho, T. Hori, H. Tozawa, and S. Kiya, xe2x80x9cBidirectional Base Station Antennas with 4-Branch Polarization and Height Diversityxe2x80x9d, Proceedings of ISAP 96, Chiba Japan, pp. 357-360, reports results which tend to corroborate the results discussed above. Cho et al. describe measured data for a number of handset antenna inclinations. The results indicate that the combined statistic of overall signal power and diversity gain favors polarization diverse antennas for mobile handset tilts greater than (approximately) 27 degrees from the vertical.
These studies, and others, use polarization-diversity that is implemented at the receiving antenna because that is, in effect, where the problem arises. In general, the transmitting antenna has no xe2x80x9cknowledgexe2x80x9d of the location, polarization, or even existence of a receiving antenna. The transmitting antenna merely creates an EM wave which radiates in many directions. A single EM wave radiated by the transmitting antenna may be received by several receiving antennas, each receiving antenna having a different polarization. Even if the transmitting antenna transmits an EM wave that is properly polarized for a particular receiving antenna, multipath effects, diffraction from objects such as buildings, and other propagation effects can rotate the polarization of the EM wave such that the polarization of the EM wave that arrives at the receiving antenna no longer matches that antenna.
Although, performance of a communication system can be improved by using a receive-only polarization-diversity, the gains are modest and may not justify the additional cost and complexity of implementation. Moreover, implementing receive-only diversity in the base station only improves the communication path from the handset to the base station (the uplink path). Polarization-diversity in the base station receiving antenna does nothing to improve the communication path from the base station to the handset unit (the downlink path). Thus, the benefits of base station diversity are one-sided. In many communications systems, there is little benefit to increasing the uplink performance if downlink performance is not similarly increased, and vice versa.
Two-way polarization diversity can be implemented by building a handset unit with a polarization-diverse receiving antenna. Unfortunately, as discussed above, implementing antenna diversity in the handset unit is typically not practical due to problems related to cost, weight, size, and complexity.
The present invention solves these and other problems by disclosing polarization diversity for base station antennas under both receive and transmitting conditions. Since the base station provides polarization diversity in both transmit and receive modes, no polarization diversity is needed in the handheld unit. Even though the handheld unit does not provide polarization diversity, a duplex communication system, that uses polarization diversity for both the uplink and the downlink is provided, because the base station provides polarization diversity for the uplink and the downlink paths. By installing the two-way diversity at the base station, the overall cost of implementing diversity is reduced because one base station can typically serve many handsets.
The base station antenna determines the polarization state of signals received from a remote unit, such as a handheld unit, using a polarization diverse antenna system. The base station then transmits using the same polarization. In a preferred embodiment, this system is used with a time-division duplex system.
In one embodiment the base station has a polarization diverse antenna comprising several antenna elements configured to receive EM waves having different polarization states. In one embodiment the antenna elements are configured to receive EM waves that are cross-polarized. In another embodiment, a first antenna element is configured to receive horizontally polarized waves and a second antenna element is configured to receive vertically polarized waves.
During receive mode, the power and phase of the output signal from each antenna element is measured. A diversity receiver combines the output signals to achieve diversity gain. Upon going into transmit mode, the base station transmitter weight the antenna output powers in a ratio corresponding to their received power measurements, and with relative phases which are reversed from the received phases. By so doing, the base station effectively tracks the polarization of the signal transmitted by the mobile unit such that the same polarization state is used for both transmit and receive functions. The base station adopts a transmit polarization that is better suited to the polarization of the antenna on the handset unit, regardless of the orientation of the handset.
In another embodiment, predictive algorithms are used to predict a polarization state for the next re-transmission.
The present invention may be used in many wireless systems including, for example, DECT, PHS, PACS-UA, PACS-UB, PWT, PWT(E), and in third-generation wireless systems, such as the proposed CDMA/TDD system.